The Crystalline Units:

A typical feature of the Kumaon Lesser Himalaya is the occurrence of scattered outcrops of older crystalline rocks in the form of klippe which are believed by most workers to be the remnants of an immense thrust sheets or nappe of rocks pushed southwards from the Central Axial Crystalline Zone of the Higher Himalaya over the younger sedimentary formations of the Lesser Himalaya (Heim and Gansser, 1939; Gansser, 1964; Ghose et al., 1974; Misra and Bhattacharya, 1976; Bhanot et al., 1977).  The crystallines consist of Almora, Baijnath, Askote, Chiplakot, and Satpuli units (Figure 1) and are characterised by a set of general features:

  • Rocks constituting the crystalline units are metamorphosed equivalents of argillaceous sediments and display the phenomenon of inverse metamorphism in which rocks with low metamorphic grades occur at the base, while higher grade rocks lie higher up the sequence.
  • Each unit consists of meso- to kata-grade of metamorphic rocks together with granites and migmatites.
  • Granitic gneisses and porphyries occur in the core of all these synclinal crystalline units.
  • Each of these units is dispersed in a synform, though severed by later folds, faults, and thrusts.
  • Of the various younger sedimentary units, it is usually the Berinag Formation that underlies the crystallines.
  • Metavolcanic rocks are generally seen at the contact of the crystallines with the sedimentaries.
  • A zone of chlorite schists is quite common along the base of all these crystalline units indicating retrogression (to epi-grade).

The most extensive crystalline unit is the Almora-Dudhatoli Crystallines.  It occurs as roughly ESE-WNW trending synform and is delineated along the two flanks by what are known as the South Almora Thrust and the North Almora Thrust.  The South Almora Thrust separating the Almora-Dudhatoli Crystallines from the underlying Outer Sedimentary Belt is a very low dipping feature.  The convergence of overlying and underlying formations of almost similar lithological units has made its recognition and delineation quite difficult.  Some workers (Kumar et al., 1974; Saxena and Rao, 1975) deny its very existence.  Detailed structural studies by Merh and Vashi (1965, 1976), Vashi and Merh (1974), Ghose (1973), and other workers have conclusively established the existence of the South Almora Thrust which joins up with the North Almora Thrust at the northwestern closure of the vast plunging syncline in the western Nayar valley, west of Dudhatoli massif. 

Valdiya (1978) and Bhattacharya (1980) are of the opinion that the metasedimentary sequence of the southern limb of the Almora synform is constituted of two thrust sheets, the upper one is called the Almora Nappe and the lower the Ramgarh Nappe (Pande, 1950) overlying the Outer Sedimentary Belt.  The Ramgarh Nappe is composed of mildly metamorphosed flyschoid rocks.  The delimiting Ramgarh Thrust is not discernible in the northern flank, being overlapped by the rocks of the Almora Nappe with which the Ramgarh Nappe forms an imbricating pair.  However, Raina and Dungrakoti (1975) deny the existence of the Raingarh Thrust.

Notwithstanding the opinion of most students of Himalayan architecture, some workers - Saxena (1974), Saxena and Rao (1975), Misra and Sharma (1972), Misra et al., (1973), and Bhattacharya (1980) suggest that the crystallines do not occur as nappes with their roots in the Central Crystallines - instead they are autochthonous in nature with their root zone along the North Almora Thrust, and their present disposition is due to vertical uplift. 

The various structural and tectonic considerations, supported by the correlation of radiometric age determinations of the granitic gneisses of the Central Crystalline Zone with those of the Crystalline sheets of the Lesser Himalaya (Bhanot et al., 1977), indicate that a great nappe of crystalline rocks was pushed southwards over the sedimentary rocks of the Kumaon Himalaya.  This crystalline sheet possibly extended as far as the Main Boundary Fault because the Satpuli Klippe is exposed very close to it (Figure 1).  Saxena (1977) considers a Permo-Carboniferous age for this thrusting.  Raina et al., (1980) favour a Middle Miocene age.

Geology

Structure

Mineralization

Wallrock Alteration

Lineaments

   


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